1. Field of the Invention
The present invention relates to a digital wave shaping circuit, a frequency multiplying circuit, and an external synchronizing method and external synchronizing circuit, which produce digital signals, the phase of which is identical to the phase position of external trigger signals with the frequency thereof identical to the frequency of digital input signals.
2. Description of the Related Art
Generally, the interruption frequency of semiconductor active elements is definite, and the maximum usable frequency is subject to the limitation thereof.
For example, in FIG. 21, the wave forms of digital signals (1) and (2) have the same repeating frequency, f. However, because the ratio of the upside H level period TA to the downside L level period TB in the wave form of signal (2) is not equal to 1:1, whereas the ratio in the wave form of signal (1) is equal to 1:1 (50% duty cycle), the apparent (virtual) frequency of signal (2) is greater than the frequency of signal (1). The apparent frequency is important in the case of an IC of a semiconductor active element receiving an input signal having a wave form of 50% duty cycle, signal (1), and operating at the extreme upper limit of its operational frequency. If the duty cycle of the input waveform is reduced to a smaller duty cycle (signal (2)), then the apparent frequency of the input wave form exceeds the operational frequency of the IC, and the IC is unable to respond to the apparent frequency of the input signal.
Therefore, it is preferable that digital signals have a wave form in which the ratio of period TA to period TB is equal to 1:1 (50% duty cycle). In other words, if the digital signals to be handled by semiconductor active elements have wave forms of 50% duty cycle, then the elements are able to handle digital signals of higher frequency.
In view of these points, a consideration is given of a digital wave shaping circuit, a frequency multiplying circuit, and an external synchronizing circuit.